Dammit - I'm always late to these!!! Prolly cuz I'm still working part-time. Great posts - yup - the Air Force is the way to go for certain people. We tend not to be as structured, although it depends on your AFSC. As for responsiblity and leadership, again, it depends on your AFSC. Now, as for the Marines - I must say I respect them immensely. They usually are the first in and the last out. They have to have uber discipline or all hell would break lose in the middle of all hell. As for the Air Force, well, if you aren't a pilot then you are helping a pilot get off the ground in one way or another. And yes, they are at the tip of the Air Force spear, whereas the Marines units are the tip of their spear.
With respect to the OPs dilemma, many others have posted great advice. I think what is important is to have the person making the decision figure out what type of person they are and environment they like to work in - of course, one needs to experiment to find out, however, as his parents, you can give some guidance. It seems as though he might need outside structure or discipline to enforce it upon his internal and external behavior. As a commander I marveled at how many people needed the military structure to function in their lives. If they 'screwed up', the military would impose the discipline on them externally. At work the other day, I had someone describe those with 'frightened minds' and those who needed the world to seriously knock them upside the head to get things done. He likened the Air Force culture to a 'frightened mind' one. The Air Force is also one of the more technically oriented services - although the other services also have their technical pockets.
As for amenities, yes, the Air Force has worked on and prides itself on providing good facilities for their personnel and dependents. I currently work at an Army garrison and the difference between that place and the Air Force bases I've worked on is immense. However, at the larger Army posts, one does see similar amenities to those in the Air Force. I've also seen some very nice Navy facilities. I've never seen a Marine facility (do they even have those? they pretty much are in the field or are an office somewhere on another base/post/etc).
So after rambling a bit - your stepson needs to want this themselves; otherwise it will be hell for all concerned (although the Marine Col I spoke with about my stepson said "we'd take him and instill some discipline" with all the confidence of having worked with unruly young men for years). With that being said, I consider my now 24 years of combined active and reserve service experience some of the most rewarding in my life. The skills I learned as an Air Force officer immensely enhanced my civilian career and helped me gain confidence in areas I never would have imagined early on. In my civilian career, I am usually tapped for leadership positions and challenging assignments and I partly attribute that the expectations and tasks I had as an officer in the United States Air Force. As I said to many of my colleagues both military and civilian: If you can make it in the military corporate culture which is one of the more controlling and authoritative cultures (even to your personal life), then you can survive and thrive in most corporat cultures.
Good luck with your son - sigh - it takes much longer for some to figure out why they are here.